A homopolymer or a copolymer containing (meth)acrylic acid and/or a salt thereof as a monomer component (hereinafter referred to as a (meth)acrylic acid-based polymer), has excellent adhesive properties, thickening properties, water absorption properties, and flocculating properties and the like, and by the use of such properties, the (meth)acrylic acid-based polymer has been widely used as a thickening agent, a self-adhesive for a cataplasm, an intensifier for paper, a curing agent, a flocculating agent, a dehydrating agent and a soil conditioning agent and the like. Above all, a (meth)acrylic acid-based polymer having a low degree of neutralization made of poly(meth)acrylic acid or (meth)acrylate whose degree of the neutralization is not more than 50 mol %, shows particularly outstanding performance in the above-mentioned uses. When a (meth)acrylic acid-based polymer is employed for such a use, it is supplied in the form of a powder or a granule in many cases, in taking into consideration of easy blending, convenient handling and convenient transportation.
By the way, methods of producing a (meth)acrylic acid-based polymer includes bulk polymerization, suspension polymerization, emulsion polymerization, slurry polymerization, solution polymerization and the like, but the methods used most extensively among them are the solution polymerization and the slurry polymerization.
When a slurry polymerization method is employed, a polymerizable monomer mainly comprising (meth)acrylic acid or a salt thereof and a radical polymerization initiator are polymerized in an organic solvent which is a poor solvent of the polymerization product, to give a (meth)acrylic acid-based polymer as a precipitate. On the other hand, when a solution polymerization method is employed, a polymerizable monomer mainly comprising (meth)acrylic acid or a salt thereof is dissolved in a solvent such as water, a radical initiator is added thereto and they are appropriately heated, if necessary, so that a polymerization reaction is carried out to give a (meth)acrylic acid-based polymer as a viscous solution.
The precipitate of a (meth)acrylic acid-based polymer obtained by slurry polymerization can be easily powdered by vacuum drying and the like, however, this process has been presenting a problem of requiring investment on new plants and equipments for removing and recovering the organic solvent, and for preventing explosion, and a problem as to the safety of the organic solvent remaining in the polymer. Another drawback is that the molecular weight of the resultant polymer can be controlled only in a relatively small range.
On the other hand, the solution of a (meth)acrylic acid-based polymer obtained by solution polymerization, particularly by aqueous solution polymerization, having a high degree of neutralization exceeding 50 mol %, has low adhesive properties and low film forming properties, therefore the solvent can be relatively easily evaporated by a known drum dryer, a ventilation type hot air dryer and the like, to give a polymer in the form of a powder. However, a solution, particularly an aqueous solution containing a (meth)acrylic acid-based polymer having a low degree of neutralization, which is not more than 50 mol %, has such high adhesive properties that, after the drying process the adhered polymer on the dryer is difficult to be detached, and since the film forming properties are high, drying of the surface layer, which is the plane through which the solvent evaporates, causes skinning to prevent the incorporated solvent from evaporating. One possible way to remove the incorporated solvent on the condition that skinning is to crush it during the drying process to expose the interior which is not yet dried, however, the (meth)acrylic acid-based polymer having a low degree of neutralization has a low glass transition temperature, and cannot be easily crushed. If the drying temperature is raised excessively, the surface layer becomes over-heated to allow branching reaction, cross-linking reaction and the like, that results in lowering of the solubility in water or generating a water insoluble product, or to allow depolymerization which will give a bad effect on the thickening properties of the polymer when the polymer is dissolved again.
The present inventors have paid attention to the above-mentioned problems of the prior art and have been studying on a method of drying a viscous solution, particularly an aqueous solution, containing a (meth)acrylic acid-based polymer having a low degree of neutralization which is not more than 50 mol %, by efficiently removing water from the solution, and have already suggested a drying method disclosed in Japanese Patent Laid Open Hei 6-41313 (Publication date: Feb. 15, 1994) as a part of the result of the study. The invention disclosed in the above specification is a method of drying an aqueous solution containing a (meth)acrylic acid-based polymer which has a degree of neutralization that is not more than 50 mol % and a water content of more than 45%, comprising first step wherein pre-heating is carried out to a temperature not more than about 70.degree. C., second step wherein the overall water content is lowered to around 40% while the surface temperature is controlled to 70.degree.-150.degree. C., and the difference between the surface temperature and the internal temperature is controlled to be not more than 50.degree. C., and third step wherein the overall water content is lowered to be not more than 10% while the surface temperature and the internal temperature are kept at 80.degree.-170.degree. C. and the difference between the surface temperature and the internal temperature is controlled to be not more than 50.degree. C. By employing this method, the water content of a (meth)acrylic acid-based polymer can be efficiently reduced to give a dried substance while the change of properties of a (meth)acrylic acid-based polymer by heat is prevented.
This process, however, requires three-step drying processes and exact control of the surface temperature, and the internal temperature of the substance to be dried, and further requires even exact control of the difference between the surface temperature and the internal temperature, and the operation including the temperature control is troublesome.